1. Field of the Invention
The present invention relates, generally, to hydraulic valves and, more specifically, to hydraulic valves for use in the control modules of automatic transmissions.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle.
Most automatic transmissions are controlled by hydraulically actuating the various components within the transmission. Thus, it is important to provide a stable hydraulic pressure for these devices. To this end, a pump is employed to provide pressurized hydraulic fluid for the control and actuation of the transmission. In addition, the clutches and gear assemblies are lubricated and cooled by a secondary flow of the hydraulic fluid. Typically, the pump is mechanically driven by a power take-off from the engine. Thus, the hydraulic pressure delivered from the pump increases as the pump speed increases in response to an increase in engine speed. Since hydraulically actuated devices respond in a predetermined and precise manner for the given pressure applied to actuate them, inaccurate control of the hydraulic pressure causes inaccurate operation and control of the automatic transmission.
To address the changes in the hydraulic pressure delivered by the pump as engine speed changes, automatic transmissions typically include a hydraulic control module that employs a plurality of hydraulic valves. A number of the valves of the type commonly employed in the hydraulic circuit of the transmission may include a valve member slidably disposed within a valve body. The valve member moves back and forth over the various ports of the valve body to direct and control the fluid flow between the ports. The valve bodies are typically received in corresponding bores formed in the control module.
The hydraulic valves may be fixedly mounted to the control module in a number of ways. For example, the valve assembly may employ a flange that is bolted to the control module using fasteners. Alternatively, the valve may employ O-rings or other seals disposed about the outer circumference of the valve body and that sealingly engage with the inner diameter of the bore formed in the control module.
While these mounting systems have generally worked for their intended purpose, certain disadvantages are presented when using these mounting strategies. For example, flange-mounted valves suffer from the disadvantage of high-costs associated with the number of fasteners needed to mount a plurality of valves to the control module. In addition, there is an ongoing need to minimize the space or “packaging” associated with transmission components, including the control module. However, both flange-mounted and O-ring sealed hydraulic valves suffer from the disadvantage that they are relatively long and thus present a larger profile. This requires more space to mount them to the control module, and thereby increases the “package” of the control module.
Partly due to these deficiencies, hydraulic valves were developed that have valve bodies that are “slip fit” into the bores formed in the control module. The tolerance between the valve body and the bore are designed such that the valve assemblies are fixedly retained in the bore. This “slip fit” mounting approach allows for a shorter valve body and thus a lower profile valve assembly. This also allows the size or “package” of the control module to be reduced.
Unfortunately, valve assemblies that are slip-fit mounted to control modules used in automatic transmissions suffer from the disadvantage that they tend to leak around the outer diameter of the valve body in the bore. Leakage of this type is not insignificant when the number of hydraulic valves employed for a transmission control unit is taken into consideration. Thus, larger capacity pumps have been employed in order to account for this defect. However, larger capacity pumps have higher costs to manufacture, and require more energy to operate. Alternatively leakage may be reduced by employing tighter clearances and less tolerance between the valve body and the bore in the control module. However, this requires additional machining and also increases the cost of manufacturing the valve and the control module. In view of the ongoing need to reduce costs and improve fuel economy, the slip-fit approach to mount a hydraulic valve to a transmission control module has been deemed problematic.
Thus, there remains a need in the art for a hydraulic valve that may be quickly and easily slip-fit mounted to a hydraulic control module for an automatic transmission at low cost, and that has a low profile and facilitates a smaller package for the control module. In addition, there remains a need in the art for a slip-fit mounted hydraulic valve having improved leakage characteristics so that smaller capacity pumps having lower energy requirements may be used to pressurize the transmission's hydraulic circuit.